path.r

# Path functions ported to R from Python os.path module

path_join = function(a, ...) {
  p = list(...)

  path = a
  for (b in p) {
    b_wins = 0  # set to 1 iff b makes path irrelevant
    if (path == "") {
      b_wins = 1
    }
    else if (isabs(b)) {
      # This probably wipes out path so far.  However, it's more
      # complicated if path begins with a drive letter:
      #   1. join('c:', '/a') == 'c:/a'
      #   2. join('c:/', '/a') == 'c:/a'
      # But
      #   3. join('c:/a', '/b') == '/b'
      #   4. join('c:', 'd:/') = 'd:/'
      #   5. join('c:/', 'd:/') = 'd:/'
      if (substring(path, 2, 2) != ":" || substring(b, 2, 2) == ":") {
        # Path doesn't start with a drive letter, or cases 4 and 5.
        b_wins = 1
      }
      # Else path has a drive letter, and b doesn't but is absolute.
      else if (nchar(path) > 3 || (nchar(path) == 3 && substring(path, nchar(path)) %nin% c('/', '\\'))) {
        # case 3
        b_wins = 1
      }
    }

    if (b_wins) {
      path = b
    }
    else {
      # Join, and ensure there's a separator.
      if (substring(path, nchar(path)) %in% c('/', '\\')) {
        if (b != '' && substring(b, 1, 1) %in% c('/', '\\')) {
          path = paste(path, substring(b, 2), sep='')
        }
        else {
          path = paste(path, b, sep='')
        }
      }
      else if (substring(path, nchar(path)) == ":") {
        path = paste(path, b, sep='')
      }
      else if (b != '') {
        if (substring(b, 1, 1) %in% c('/', '\\')) {
          path = paste(path, b, sep='')
        }
        else {
          path = paste(path, '\\', b, sep='')
        }
      }
      else {
        # path is not empty and does not end with a backslash,
        # but b is empty; since, e.g., split('a/') produces
        # ('a', ''), it's best if join() adds a backslash in
        # this case.
        path = paste(path, '\\', sep='')
      }
    }
  }
  return(path)
}

isabs = function(s) {
  s = splitdrive(s)[2]
  return(s != '' && substring(s, 1, 1) %in% c('/', '\\'))
}

splitdrive = function(p) {
  if (substring(p, 2, 2) == ':') {
    return(c(substring(p, 1, 2), substring(p, 3)))
  }

  return(c('', p))
}